It turns out our fast path for -w/--word-regexp wasn't quite correct in
some cases. Namely, we use `(?m:^|\W)(<original-regex>)(?m:\W|$)` as the
implementation of -w/--word-regexp since `\b(<original-regex>)\b` has
some unintuitive results in certain cases, specifically when
<original-regex> matches non-word characters at match boundaries.

The problem is that using this formulation means that you need to
extract the capture group around <original-regex> to find the "real"
match, since the surrounding (^|\W) and (\W|$) aren't part of the match.
This is fine, but the capture group engine is usually slow, so we have a
fast path where we try to deduce the correct match boundary after an
initial match (before running capture groups). The problem is that doing
this is rather tricky because it's hard to know, in general, whether the
`^` or the `\W` matched.

This still doesn't seem quite right overall, but we at least fix one
more case.

Fixes #2574